A liquid crystal display (LCD) has advantages of thin shape, light weight and low consumption of electric power, compared with a cathode ray tube (CRT). The liquid crystal display comprises a liquid crystal cell and two polarizing elements arranged on both sides of the liquid crystal cell. The liquid crystal cell comprises a pair of substrates, rod-like liquid crystal compounds and an electrode layer. The rod-like liquid crystal compounds are provided between the substrates. The electrode layer has a function of applying a voltage to the rod-like liquid crystal compound. Each of the substrates has an orientation layer, which has a function of aligning the rod-like liquid crystal compound.
An optically compensatory sheet (a phase retarder) is usually provided between the liquid crystal cell and the polarizing elements to remove color from an image displayed on the liquid crystal cell. A lamination of the optically compensatory sheet and the polarizing element (polarizing membrane) functions as an ellipsoidal polarizing plate. A stretched birefringent film has usually been used as the optical compensatory sheet.
It has been proposed to use an optical compensatory sheet comprising an optically anisotropic layer containing discotic compounds on a transparent substrate in place of the stretched birefringent film. The optically anisotropic layer is formed by fixing aligned discotic compounds. The discotic compound usually has a large birefringence. Accordingly, an optical compensatory sheet obtained by using the discotic compound has a specific optical characteristic that cannot be obtained by the conventional stretched birefringent film. The optical compensatory sheet using the discotic compound is described in Japanese Patent Provisional Publication No. 6(1994)-214116, U.S. Pat. Nos. 5,583,679, 5,646,703 and German Patent Publication No. 3,911,620A1.
U.S. Pat. Nos. 4,583,825 and 5,410,422 disclose a liquid crystal display of a bend alignment mode having a liquid crystal cell in which rod-like liquid crystal compounds are aligned symmetrically. The alignment of an upper liquid crystal compound is essentially antiparallel to the alignment of a lower liquid crystal compound. The liquid crystal cell of the bend alignment mode has a self-optically compensatory function because of the symmetrical alignment. Therefore, the bend alignment mode is also referred to as an optically compensatory bend (OCB) mode. The liquid crystal display of the bend alignment mode has an advantage of a rapid response.
A liquid crystal cell of a homogeneous alignment mode has also been proposed. In the liquid crystal cell of the homogeneous alignment mode, rod-like liquid crystal compounds are essentially horizontally aligned while not applying voltage to the cell, and are essentially vertically aligned while applying voltage to the cell. The liquid crystal cell of the homogeneous alignment mode has recently been used in a liquid crystal display of an electrically controlled birefringence (ECB) type. The liquid crystal display of the ECB type is described in Japanese Patent Provisional Publication No. 5(1993)-203946.
The bend alignment mode and the homogeneous alignment mode are characterized in a wide viewing angle and a quick response compared with conventional liquid crystal modes (TN mode, STN mode). However, a further improvement is still necessary to be comparable with CRT.
It might be considered that an optical compensatory sheet is used to improve the liquid crystal display of the bend alignment mode or the homogeneous alignment mode in the same manner as in the conventional liquid crystal displays. However, the known optical compensatory sheet of a stretched birefringent film used in a conventional liquid crystal display is not effective in the liquid crystal display of the bend alignment mode or the homogeneous alignment mode. Particularly, the known stretched birefringent film shows a poor optical compensatory effect on the display of the bend alignment mode or the homogeneous alignment mode.
As is described above, an optical compensatory sheet comprising an optically anisotropic layer containing discotic compounds on a transparent substrate has been proposed in place of the stretched birefringent film. Japanese Patent Provisional Publication No. 9(1997)-197397 (U.S. Pat. No. 5,805,253) and International Publication No. WO96/37804 (European Patent Publication No. 0783128A) disclose a liquid crystal display of a bend alignment mode, which has an optical compensatory sheet containing discotic compounds. The liquid crystal display of the bend alignment mode has been remarkably improved in the viewing angle by using the optical compensatory sheet containing discotic compounds.